Image mask substrate for X-ray semiconductor lithography

ABSTRACT

Fine, sub-micron line features and patterns are created in a sensitized layer on a semiconductor wafer by a source of X-ray radiation. The X-ray source emits very low wavelength radiation along a path towards a sensitized surface of a semiconductor wafer. An image mask substrate is disposed in the path of the radiation, and is provided with a patterned opaque material on a surface of a substrate thereof. The substrate is formed of beryllium, which is robust and has a thermal coefficient of expansion closely conforming to that of common image mask carriers. Further, a wide variety of opaqueing materials adhere well to the beryllium substrate, and the substrate is relatively insensitive to moisture. The image mask is spaced sufficiently close to the wafer that radiation passing through the mask forms a corresponding pattern in the surface of the wafer. For X-ray radiation, the opaqueing material is gold, tungsten, platinum, barium, lead, iridium, rhodium, or the like.

This a continuation of application Ser. No. 08/056,334, filed on Apr.30, 1993, and now abandoned.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to techniques for manufacturingsemiconductor devices and, more particularly, to techniques for formingpatterned features on a semiconductor device.

BACKGROUND OF THE INVENTION

Photolithography is a common technique employed in the manufacture ofsemiconductor devices. Typically, a semiconductor wafer is coated with alayer of light sensitive resist material (photoresist). Using apatterned mask or reticle, the wafer is exposed to projected light froman illumination source, typically actinic light, which manifests aphotochemical effect on the photoresist, which is ultimately (typically)chemically etched away, leaving a pattern of photoresist "lines" on thewafer corresponding to the pattern on the mask or reticle. The patternedphotoresist on the wafer is also referred to as a mask, and the patternin the photoresist mask replicates the pattern on the image mask (orreticle).

As used in the main, hereinafter, with respect to semiconductorlithography, the term "upstream" means towards the illumination orradiation source, and "downstream" means away from the illuminationsource (or, towards the wafer). For example, a lens in the illuminationpath of photolithographic apparatus has an upstream side facing theillumination source and a downstream side facing away from theillumination source.

FIG. 1 shows a simplified prior-art photolithographic apparatus 110 forexposing a semiconductor wafer (W), more particularly a coating thereon(e.g., photoresist), to light. An optical path is defined from left toright in FIG. 1, as viewed. Prior to exposure, the semiconductor wafer(W) typically receives on its front surface a layer of photoreactivematerial (not shown), such as photoresist.

A light source 112 emits actinic light, and may be backed up by areflector 114. Light emitted by the light source typically passesthrough a uniformizer 116, such as a "fly's eye" lens or a light pipe.

Light exiting the uniformizer 116 is represented by rays 118a, 118b, and118c, and passes through a condenser lens 120. The ray 118b representsthe optical axis of the photolithographic apparatus. The light source112, reflector 114, uniformizer 116 and condenser lens 120 form what istermed an "illuminator" which is often detachable as a unit from thephotolithographic apparatus.

An image mask 122 ("M") is disposed "downstream" of the condenser lens120, at the focal plane (point) thereof. One type of image mask used inthe photolithography process is a chromed glass or quartz plate bearingthe pattern to be projected onto the photoresist layer. Light isprojected through the image mask, and those areas of the image maskwhich are not chromed allow the light to expose the photoresist, whilethose areas of the image mask which are chromed prevent the light fromexposing the photoresist. The exposed areas of the photoresist typicallyresist chemical etching, while the unexposed areas can readily beremoved, leaving a pattern of photoresist on the surface of the wafer.

Further downstream along the light path, the rays diverge from the mask122, and pass through a "taking" (imaging) lens 124. Because of itsimaging function, the taking lens 124 must be of relatively high qualityas compared with the condenser lens 120. The mask 122 is disposed at acommon focal point of the two lenses 120 and 124.

A semiconductor wafer (W) is disposed at the "downstream" focal plane,or image plane, of the taking lens 124. Those areas of the mask (orreticle) which are not chromed allow the light to expose a photoreactivelayer (e.g., photoresist) on the surface of the wafer (W), while thoseareas of the mask which are chromed (or otherwise opaquely patterned)prevent the light from exposing the photo-reactive layer. Thephotoreactive layer is typically a photoresist material. The exposedareas of the photoresist resist chemical etching and, in subsequentprocessing, are used to form defined features on the wafer (such as on alayer of polysilicon underling the photoresist).

The resist materials used in photolithography are typically organic.Typical resist materials for visible light photolithography includemixtures of a casting solvent, such as ethyl lactate, and novolac resin(diazoquinone).

Inasmuch as the light passing through the image mask (reticle) has aninherent characteristic that induces photochemical activity in thephotoresist material, such radiation (e.g., light) is termed "actinic".

In current photolithographic apparatus, light having at least asubstantial visible content is typically employed. Visible light has afrequency on the order of 10¹⁵ Hz (Hertz), and a wavelength on the orderof 10⁻⁶ -10⁻⁷ meters.

The following terms are well established: 1 μm (micrometer) is 10⁻⁶meters; 1 nm (nanometer) is 10⁻⁹ meters; and 1 Å (Angstrom) is 10⁻meters.

Among the problems encountered in photolithography are non-uniformity ofsource illumination and point-to-point reflectivity variations ofphotoresist films. Both of these features of current photolithographyimpose undesirable constraints on further miniaturization of integratedcircuits. Small and uniformly sized features are, quite evidently, theobject of prolonged endeavor in the field of integrated circuit design.Generally, smaller is faster, and the smaller the features that can bereliably fabricated, the more complex the integrated circuit can be.

With regard to uniformity of source illumination, attention is directedto commonly-owned U.S. Pat. No. 5,055,871, issued to Pasch. As noted inthat patent, non-uniformities in the illuminating source will result innon-uniformities of critical dimensions (cd) of features (e.g., lines)formed on the semiconductor device, and the illumination uniformity ofphotolithographic apparatus will often set a limit to how small afeature can be formed. There usually being a small "error budget"associated with any integrated circuit design, even small variations inillumination intensity can be anathema to the design goals.

with regard to reflectivity of photoresist films, it has been observedthat minor thickness variations in a photoresist film will causepronounced local variations in how efficiently the illuminating light isabsorbed (actinically) by the photoresist film, which consequently canadversely affect the uniformity of critical dimensions (cd) of features(such as polysilicon lines or gates) sought to be formed in a layerunderlying the photoresist. This problem is addressed in commonly-owned,copending U.S. patent application Ser. No. 07/906,902, filed Jun. 29,1992 by Michael D. Rostoker, which discussed techniques for applying asubstantially uniform thickness layer of photoresist, and which isincorporated by reference herein.

Another, more serious problem with photolithography is one of itsinherent resolution. The cd's of the smallest features of today'sdensest integrated circuits are already at sub-micron level (a "micron"or "μm" is one millionth of a meter). Such features are only slightlylarger than a single wavelength of visible light, severely pushing thelimits of the ability of visible light techniques to resolve thosefeatures. As integrated circuit features become smaller, the demand formore nearly "perfect" optical components increases. At some point,however, such optics become impractical and inordinately expensive, oreven impossible to produce. Although the resolving power of light,vis-a-vis submicron semiconductor features is being stretched to itslimit, the ability to etch (wet, dry, chemical, plasma) features on asemiconductor wafer is not limited by wavelength.

As is well known, ultraviolet light (UV) is slightly higher (infrequency) on the electromagnetic spectrum than visible light.Typically, ultraviolet light has a frequency on the order of 10¹⁵ -10¹⁷Hz, and has a wavelength on the order of 10⁻⁷ -10⁻⁸ meters. Ultravioletlight is known to be actinic, for example with respect to skinpigmentation. Due to its shorter (than visible light) wavelength,ultraviolet light would seem to hold promise for increased resolution inintegrated circuit photolithography. However, it is difficult to findreliable, fluent sources of UV (typically vacuum UV) light. Further, theperformance of present day optics begins to degrade substantially ataround 190 nm (1.9×10⁻⁷ meters; which is towards the top of the visiblelight spectrum), and is not well suited for focusing UV light.

In contrast to visible light, X-rays have a much shorter wavelength.Typically, X-rays have a frequency on the order of 10¹⁷ -10²⁰ Hz, andhave a wavelength on the order of 10⁻⁸ -10⁻¹¹ meters. Evidently, due totheir shorter wavelength, X-rays have the inherent capability ofproviding better resolution than visible light. However, as with UVsources, there are some problems with obtaining reliable emissionsources that exhibit good fluence. The best (most intense) X-ray sources(e.g., X-ray tubes) produce X-rays in the range of 1-10 Å in wavelength,with a nominal output spectrum between 2 Å and 6 Å in wavelength.

Gamma-rays exhibit an even shorter wavelength than X-rays. Typically,Gamma-rays have a frequency on the order of 10¹⁹ -10²² Hz, and have awavelength on the order of 10⁻¹⁰ -10⁻¹² meters. Evidently, Gamma-raysprovide the potential for even better resolution than X-rays.Furthermore, gamma-ray sources providing intense streams of fluentemission are readily available, such as in the form of Cobalt-60.

In the absence of the novel viable gamma-ray and X-raysemiconductor-processing techniques disclosed herein, various techniquesfor "stretching" the resolution of UV and visible light techniques havebeen contemplated. One such technique provides a method of formingshort-channel polysilicon gates (0.6 μm polysilicon feature size). (See,for example, U.S. Pat. No. 5,139,904, issued Aug. 18, 1992 to Auda etal.) This method employs a technique of laying down a layer ofconventional photoresist over a polysilicon layer and patterning thephoto-resist to "normal" dimensions (greater than the ultimately desired0.6 μm dimension). The photo-resist pattern is then uniformly eroded inall dimensions using an isotropic (non-directional) RIE (reactive ionetching) etch process. The size of features in the photo-resist patternis carefully monitored during the etch process. When the patternfeatures are eroded to the desired size, the etch process is stopped. Ananisotropic (highly directional) etch process is used to etch awayportions of the underlying polysilicon outside of the "shadow" of theeroded photo-resist pattern (relative to a generally vertical etchdirection).

While this technique may be employed to produce small polysiliconstructures, it has the same limitations as conventional photolithographywith respect to line-to-line spacing. Because the photoresist isinitially patterned to "conventional" dimensions, it is not possiblewith such "stretched" techniques to space pattern features substantiallycloser with sufficient resolution than is ordinarily possible withconventional photolithography.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide improvedtechniques for fabricating semiconductor devices.

It is another object of the present invention to provide improvedtechniques for forming ultra-fine features on a semiconductor device.

It is another object of the present invention to provide techniques forforming features on a semiconductor device which are not limited by theresolving power of light.

It is another object of the present invention to provide waferprocessing techniques which yield improved critical dimensions (cd's) insemiconductor features.

It is another object of the present invention to provide techniquescapable of resolving smaller features (such as polysilicon or metallines).

It is another object of the present invention to provide near-fieldafocal techniques for processing semiconductor wafers.

It is another object of the present invention to provide X-raylithographic techniques.

It is another object of the present invention to provide gamma raylithographic techniques.

It is another object of the present invention to provide means for"shuttering" gamma rays or X-rays.

As used herein, the term"lithography" refers to any technique which isemployed to define features on a semiconductor wafer, for examplepatterning photoresist overlying a layer that will subsequently beetched. Generally, all of the lithography techniques discussedhereinbelow employ some form of illumination (or radiating) source.

According to the invention, lithography is performed on a semiconductordevice using electromagnetic energy of shorter, or of substantiallyshorter wavelength, than visible or UV light.

In one embodiment of the invention, X-rays are used as the illumination(radiation) source.

According to an aspect of the invention, Beryllium is used astransparent image mask substrate for imaging X-rays onto a semiconductorwafer. Beryllium has excellent transparency to X-rays, and since it is ametal itself, carriers and opaque masking materials can be readilyprovided which have similar expansion coefficients, resulting inrelatively low distortion of the mask.

According to various aspects of the invention, Gold, Tungsten, Platinum,Barium, Lead, Iridium, or Rhodium are used as opaque mask materials tobe deposited over a Beryllium substrate (image mask). All of thesematerials exhibit excellent opacity to X-rays. Further, these materialsexhibit adequate adhesion to Beryllium (the image mask substrate) andadequate environmental robustness for utility as lithographic imagemasks.

The resulting image mask (beryllium substrate with a pattern of opaquelines on a surface thereof) is suitably employed for "near field"lithography. By "near field" it is meant that the process is afocal, andby spacing the image mask close to the semiconductor wafer there islimited opportunity for the radiation passing through the image mask tospread.

In another embodiment of the invention, Gamma-rays are used as thelithographic illumination source.

According to an aspect of the invention, "base" organic resist materialsapplied to the semiconductor die (wafer) are doped either with organicor with inorganic materials (dopants) which exhibit high absorptivity togamma-rays, to enhance the sensitivity of the resist material.Preferably, the dopant is inorganic. Examples of organic dopants includepolystyrene, phenolformaldehyde, polyurethane, etc. Examples ofinorganic dopants include bromine, chromium, tantalum, gold, platinum,palladium, lead, barium, boron, aluminum and magnesium. The dopants arehighly reactive to incident gamma radiation, and produce secondaryphoton emissions of a different wavelength (longer) than that of theincident gamma rays. The organic resist base, which is not ordinarilyreactive to gamma radiation, is however highly absorptive of thesesecondary emissions (from the dopants), which are actinic with respectto the organic resist base, thereby causing the resist base to becomechemically converted. The high cross-section (absorptivity) of theorganic resist base to the secondary emissions also limits the amount of"blooming" (spreading) inherent in the secondary emissions.

According to another aspect of the invention, an organic resist materialhas an absorptive (to gamma radiation) film of material disposed on asurface thereof. The film atop the photoresist is organic or inorganic,preferably inorganic, and provides secondary emissions (photons) whichconvert the underlying photoresist. The film is termed a "secondaryresist layer". Examples of organic resist materials suitable for thesecondary resist layer include polystyrene, phenolformaldehyde,polyurethane, etc. Examples of inorganic secondary resist materialssuitable for the secondary resist layer include bromine, chromium,tantalum, gold, platinum, palladium, lead, barium, boron, aluminum andmagnesium. The secondary resist layer, when exposed to gamma radiation,produces secondary photon emissions of a different wavelength (longer)than that of the incident gamma rays. The underlying organic resistmaterial is highly absorptive of these secondary emissions, which areactinic with respect to the organic resist, causing it to becomechemically converted. The high cross-section (absorptivity) of theunderlying organic resist to the secondary emissions, and its closejuxtaposition to the overlying secondary resist film, limit the amountof "blooming" (spreading) that would otherwise be expected to beexperienced.

Other combinations of organic resist bases (or layers) either doped withhigh cross-section (to gamma radiation) dopants or underlying moreabsorptive (to gamma radiation) layers are disclosed and otherwisecontemplated.

Other aspects of the invention are directed to direct-write, afocal,lithography techniques and to means for directing, concentrating,collimating and shuttering beams of radiative energy.

According to the invention, a broad incident beam of radiation can beconcentrated and collimated, providing a very narrow, very intense beamof radiation (such as X-ray or gamma radiation) useful over a shortrange of distances as by means of a hollow, horn-shaped (e.g., conical)afocal concentrator (described extensively hereinafter). The afocalconcentrator has a tapered section and a cylindrical section. Thetapered section has a broad mouth at one end and a narrow opening at anopposite end. The cylindrical section has a diameter equal to that ofthe narrow opening, and is formed continuously therewith. A broadincident beam of radiation enters the mouth of-the tapered portion andis concentrated in the tapered portion and is collimated in thecylindrical portion to provide a collimated, intense output beam thatcan be directed onto a semiconductor wafer. In order to produce patternson the wafer, either the collimator or the wafer is moved (in two axes).Preferably, the wafer would moved and the concentrator would be fixed inposition.

According to various aspects of the invention, the concentrator may haveany of various tapered forms, including a linear, cone-shaped taper, anexponential taper, or some combination thereof. In any case, the innersurface of the afocal concentrator is highly reflective of the incidentradiation.

According to various other aspects of the invention, the afocalconcentrator may be used to collimate (thereby intensify) any of variousforms of radiation, including gamma radiation, X-ray radiation, UVlight, and visible light. In the main hereinafter, the utility of thecollimator for very short wavelength radiation that cannot be focused byconventional optics is discussed.

According to other aspects of the invention, the reflective innersurface (bore) of the afocal concentrator is formed of aluminum, nickel,or chromium. The entire collimator can be formed of a single material,or its bore can be plated.

According to the invention, a surface acoustic wave (SAW) deviceoperating as a shallow angle scattering surface, can act as a shutterfor X-ray or gamma-ray radiations. In the context of the presentinvention, such a shutter would controllably allow/prohibit thedownstream (towards the wafer, or towards the concentrator) passage ofradiation from a fluent, continuous source of radiation. A thin,reflective film of, for example, aluminum, nickel, or chromium, isdisposed over the surface of a Surface Acoustic Wave (SAW) device. Whenthe SAW device is not activated, the reflective surface is substantiallyplanar, and reflects incident radiation at an angle equal and oppositeto its angle of incidence. This beam, the position of which is highlypredictable, can be used to pattern a layer (e.g., photoresist) on asemiconductor wafer. A tightly collimated beam approaching the surfaceof the SAW (such as from the aforementioned collimator) at a knownshallow angle, will be reflected off of the reflective surface of theunactivated SAW device at a predictable angle. When the SAW device isactivated, however, the surface of the SAW device becomes distorted anddeflects or scatters the incident beam. By providing a beam stop or anaperture and positioning it such that radiation from the incident beamwill pass the beam stop (or aperture) only when reflected at an anglecorresponding to its reflection off of the planar surface of theunactivated SAW device, an effective shutter is formed. Hence, theplanar and distorted surface of the SAW device, in combination with aknife-edge, opaque beam stop or aperture, effectively functions as ashutter, turning an incident beam ON and OFF, respectively, particularlyfor very short wavelength radiation (e.g., X-rays or Gamma rays).

It is not necessary, according to the invention that the incident beambe "cleanly" reflected in any particular direction when the SAW deviceis activated (distorted surface). It is only necessary that thereflected beam be reflected from the SAW device anywhere other than pastthe beam stop or aperture when the SAW device is activated.

In a similar manner, a magnetostrictive device may be employed insteadof a SAW device, in combination with a beam stop or aperture, to form aneffective shutter mechanism. Again, the surface of the magnetostrictivedevice can selectively be made planar, to reflect incident radiationpast a beam stop or aperture, or it can be made non-planar, to divertincident radiation from passing the beam stop or aperture. As with theSAW device, the magnetostrictive device is coated with a material thatis highly reflective vis-a-vis the incident radiation. In either case,namely employing a SAW device or a magnetostrictive device, thereflective element acts as a "surface distortion device" for thepurposes of the present invention. Other devices whose surfaces mayselectively be distorted may be employed, in combination with a beamstop or aperture, to achieve a similar shuttering function.

According to various other aspects of the invention, the SurfaceAcoustic Wave or magnetostrictive shutter may be used to shutterradiation of a variety of wavelengths, including gamma-rays, X-rays, UVlight, etc. In the main hereinafter, the utility of these surfacedistortion devices in conjunction with non-visible radiation isdiscussed.

Further, according to the invention, direct-write gamma-ray lithographicapparatus is provided. An omni-directional radiation source provides asource of intense gamma-ray radiation. A suitable radiation source is aCobalt-60 pellet which passively (without any external power) radiatesintense, fluent (e.g., steady, not varying or intermittent) gamma-rayradiation. A reflector (similar to the reflector 114 discussed withrespect to FIG. 1, above) may be employed behind the Cobalt-60 pellet toimprove the directionality and intensity of the emissions from thepellet. Gamma-ray radiation from the gamma-ray radiation source enters(is incident to) a shutter device, such as the SAW ormagnetostrictive-based shutter devices described above. The shutterdevice serves to selectively gate (block or pass) the incident beam,resulting in a controlled gamma-ray beam. The controlled gamma-ray beamenters the mouth of an afocal concentrator, such as that described aboveand in greater detail with respect to FIG. 4 et seq. The afocalconcentrator narrows, intensifies and collimates the controlled beam toprovide a collimated beam. A semiconductor wafer is positioned adistance from the output of the afocal concentrator such that thecollimated beam impinges upon the surface thereof. The surface of thewafer is coated with a layer of gamma-sensitive resist, such as thatdescribed above. Preferably, the wafer is mounted to a movable carriage,by which means the wafer may be positioned such that the collimated beammay be caused to impinge on any point on the resist layer, to form apattern in the resist layer for further processing (e.g., chemicaletching). This is referred to as "direct write" lithography.

The on/off state of the collimated beam may be effectively controlled byselectively activating and de-activating the shutter device. Preferably,the distance between the wafer and the output of the afocal concentratoris approximately 5 μm. Even if the collimated beam of gamma radiation isnot perfectly collimated, by positioning the wafer so close to theoutput of the collimator, there is not much opportunity for thecollimated beam to spread out.

In an alternate embodiment of the direct-write gamma-ray lithographyapparatus described hereinabove, the positions of the shutter device andthe afocal concentrator are reversed. In other words, the gammaradiation would be collimated, then shuttered, then caused to impinge ona semiconductor wafer.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a prior-art photolithographicapparatus.

FIG. 2a is a cross-sectional view of a mask assembly, and near-fieldlithography apparatus, according to the present invention.

FIG. 2b is a cross-sectional view of an alternate embodiment of a maskassembly, and near-field lithography apparatus, according to the presentinvention.

FIG. 3a is a cross-sectional view of a gamma-sensitive resist on asemiconductor wafer, according to the present invention.

FIG. 3b is a cross-sectional view of an alternate embodiment of agamma-sensitive resist on a semiconductor wafer, according to thepresent invention.

FIG. 3c is a cross-sectional view of an alternate embodiment of agamma-sensitive resist on a semiconductor wafer, according to thepresent invention.

FIG. 3d is a cross-sectional view of an alternate embodiment of agamma-sensitive resist on a semiconductor wafer, according to thepresent invention.

FIG. 4 is a schematic diagram of an afocal concentrator, according tothe present invention.

FIG. 5a is a diagram of a surface-acoustic-wave (SAW) shutter in anunactivated (shutter open) state, according to the present invention.

FIG. 5b is a diagram of a surface-acoustic-wave (SAW) shutter in anactivated (shutter closed) state, according to the present invention.

FIG. 5c is a magnified (more detailed) view of a portion of FIG. 5b.

FIG. 6a is a side view, partially in cross-section, of an embodiment ofa short-wavelength, afocal, direct-write, lithography system, accordingto the present invention.

FIG. 6b is a side view, partially in cross-section, of an alternateembodiment of a short-wavelength, afocal, direct-write, lithographysystem, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, electromagnetic radiation of shorter, or ofsignificantly shorter, wavelength than visible light is used forlithography of integrated circuits. Given the inherent resolutionproblems associated with conventional visible light and near-visiblelight photolithographic techniques (discussed hereinabove), the use ofshorter wavelength radiation sources is highly desirable. However, dueto the failure of conventional optics to perform at these shortwavelengths, it is necessary to employ near-field or direct-write,afocal imaging techniques (non-focusing or non-converging optically)with short wavelength radiation sources. In this manner, by avoiding theinherent resolution problems associated with the relatively longwavelengths of light, finer (smaller) features can be defined on asemiconductor wafer. For example, fine lines can be patterned in a layerof photoresist material on the wafer and, subsequently, lines can beetched into a layer underlying the photoresist. The invention takesadvantage of the situation that the apparent `resolutions` (if you will)of etching techniques are typically much (e.g., orders of magnitude)finer than the resolution of light. Generally, "resolution" is theability of a given medium to create patterns on another medium. As usedherein, the term "lithography" means any technique of creating patternson a semiconductor wafer (or on a photoresist layer on the wafer). Byproviding higher (than light) resolution lithography techniques, theinvention affords the opportunity to create finer, more densely packedfeatures and devices on a semiconductor device. For example, moretransistors can be formed on a die of given area, and more conductivelines (e.g., polysilicon or metal) can be provided in a given area. Thesize (width) of conductive lines is a measure of process resolution(sometimes called "process geometry"). Present photolithographictechniques, limited as they are by the relatively long wavelengths oflight, are limited to approximately 0.5 μm. By using the techniquesdisclosed herein, which involve employing radiation of shorter orsignificantly shorter wavelengths than light, conductive lines that aresmaller, and much smaller than 0.5 μm can be created-on semiconductordevices. For example, lines having a width of less than "w" microns canreadily be fabricated on semiconductor devices, where "w" is below 0.5,0.4, 0.3, 0.2, 0.1, and smaller. Current densities in such "fine" linesevidently needs to be controlled (or limited). In order to increase thecurrent-carrying capability of such fine lines, it is also contemplatedherein that the height (above the surface of the wafer) of such linesmust be maximized For example, a line having a width of 0.1 μm can beformed that has a height "h" of 0.2 μm, in which case the h:w "aspectratio" of the line would be on the order of 2:1. According to an aspectof the invention, which is provided herein mainly as a "rule of thumb",fine conductive lines have a height:width (h:w) aspect ratio of at least"x", where "x" is 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 17:1, 1.8:1, 1.9:1,2:1, 2.5:1, 3.0:1, 3.5:1, 4.0:1, 4.5:1 or 5.0:1. Preferably, "x" is atleast 2.0:1.

In contrast to a technique that provides only narrow lines, withoutallowing for increased line density (see, e.g., U.S. Pat. No. 5,139,904,described above), the present invention allows for both finer lines andfor packing fine lines close together.

In one embodiment of the invention, integrated circuit (semiconductor)lithography is performed using X-ray emissions as the actinic source. Asdescribed hereinabove, the use of short-wavelength radiation sources,such as X-rays, for semiconductor lithography is less wavelengthresolution-limited than light for fabricating small-geometry (sub-microncd) integrated circuits. The short wavelength (10⁻⁸ -10⁻¹¹ meters) ofX-ray radiation is well suited to providing the resolution required forthe formation of very small sub-micron semiconductor features insemiconductor devices.

Some efforts have been made to use X-ray lithography. However, suchefforts are plagued with difficulties. Present image mask substratematerials include silicon carbide, polyimide, and silicon dioxide. Thesemasking substrate materials suffer from a number of shortcomingsrelative to X-ray lithography. Among these are:

1) present X-ray image mask substrates exhibit extremely poortransparency to X-rays, yielding masks which provide poor contrast;

2) the poor transparency of present X-ray mask substrate materialsforces the use of extremely thin substrates, resulting in very fragileimage masks;

3) present X-ray mask substrate materials are subject tohumidity-induced distortions, yielding image masks of poor stability,and causing unpredictable critical dimensions and feature positions in aresist material exposed by such image masks;

4) the expansion and adhesion characteristics of the opaque materialspatterned on present mask substrates result in pattern-dependentdistortions of the thin image mask substrates; and

5) present X-ray image mask substrate materials have poor "windows oftransparency" (ranges of wavelengths for which they are transparent)which do not include the wavelength of the most desirable (shortest)X-ray wavelengths and the most intense available X-ray sources.

Further, an image mask formed from these substrate materials is subjectto an overall distortion within the "carrier" to which it is mounted dueto different rates of thermal expansion between the image mask and thecarrier. When heated, present X-ray image masks distort to anunacceptable level, thereby requiring exotic processing techniques.

According to the invention, Beryllium metal (chemical symbol Be) is usedas an X-ray image mask substrate. Beryllium has many desirablecharacteristics which make it quite suitable for use as an X-ray imagemask substrate. These desirable qualities of beryllium include:

1) good transparency to X-rays;

2) viability of formation into thicker (than present) mask substratesdue to its good X-ray transparency;

3) insensitivity to moisture-induced distortion;

4) excellent Young's modulus to resist distortion;

5) expansion characteristics more compatible with those of availableopaque masking materials and carrier materials.

6) a wide window of transparency, permitting the use of high-intensityX-ray sources of the shortest wavelengths possible.

According to the present invention, a Beryllium substrate is used as aplanar image mask substrate upon which a patterned layer of X-ray opaquematerial is disposed.

In conjunction with the use of a beryllium substrate for the X-rayexposure mask, the following materials make excellent X-ray opaqueingmaterials, for forming patterns on the image mask: gold (chemical symbolAu), tungsten (chemical symbol W), platinum (chemical symbol Pt), barium(chemical symbol Ba), lead (chemical symbol Pb), iridium (chemicalsymbol Ir), and rhodium (chemical symbol Rh). These materials haveexcellent opacity to X-rays, are relatively insensitive to moisture andare highly corrosion resistant. Further, these materials will adhereadequately to a beryllium substrate, thereby making the combination ofthese materials patterned on a beryllium substrate ideal for X-raylithography of semiconductor devices. The thickness of the maskingmaterial can be empirically determined for each material (e.g., forgold) and based upon the parameters of the specific X-ray sourceemployed. The mask pattern should be substantially opaque to the X-rayemissions, thereby providing high contrast on the image mask.

Since, like beryllium, these masking materials (e.g., gold) are allmetals, their thermal coefficients of expansion are highly compatible(substantially equal) to the thermal coefficient of expansion of theberyllium image mask substrate. This serves to reduce the amount ofthermally-induced pattern-dependent mask distortion as compared to thatexperienced with present mask and substrate materials. The insensitivityof the inventive combination of masking materials and substrate tohumidity serves to substantially eliminate pattern-dependent maskdistortion due to humidity.

According to the present invention, an X-ray mask is formed by providinga substantially planar beryllium substrate, and disposing upon thesubstrate a patterned opaque (to X-rays) layer of gold, tungsten,platinum, barium, lead, iridium or rhodium. The patterned layer may bedisposed on either the "upstream" (towards the X-ray source) side of theberyllium substrate or on the "downstream" (towards the semiconductorwafer) side of the substrate.

The mask is positioned in close proximity "upstream" of a sensitizedwafer (a wafer with a layer of X-ray sensitive resist). The wafer isthen exposed to an upstream X-ray source through the mask, causing adownstream "shadow" image of the mask to be formed on the surface of theresist. The close proximity of the image mask to the substratesufficiently avoids dispersion of the illuminating radiation so that a"copy" of the image mask pattern is imaged onto the sensitized (e.g.,with a layer of photoresist) wafer.

The X-ray radiation is actinic with respect to the resist, and causesexposed areas of the resist to become chemically converted. Afterexposure to X-ray radiation, the unconverted areas of the resist (thoseareas of the resist which were "shadowed" by the mask) are chemicallyremoved. (This described a "negative" mode of resist development. It isalso possible to employ a "positive" resist chemistry, for which onlythe exposed areas of the resist are chemically removed).

FIG. 2a is a cross-sectional view of semiconductor lithographyapparatus, according to the present invention, showing an X-ray maskassembly 200a comprising an image mask 215a and a carrier 210 for theimage mask 215a, according to the invention. A patterned opaque layer230 of an X-ray masking material (e.g., gold), as described hereinabove,is disposed on the "upstream" surface (left hand surface, as shown inthe Figure) of a planar beryllium substrate 220 to form the image mask215a. The image mask 215a is fastened to the carrier 210 in any suitablemanner. Preferably, the carrier 210 has a coefficient of expansionsimilar, or substantially identical, to that of the beryllium substrate220. This is quite feasible since the beryllium itself is a metal. Forexample, the carrier 210 is also made of beryllium so that its expansioncharacteristics are identical to that of the beryllium substrate 220,effectively eliminating any thermally induced distortion of the mask215a.

The image mask 215a is positioned a "near field" distance "d" upstreamof a front (left, as viewed in the Figure), sensitized (e.g., withphotoresist) surface of a semiconductor wafer "W" such that the plane ofthe image mask 215a is substantially parallel to the front surface ofthe wafer "W". The sensitized wafer "W" has a layer of X-ray sensitiveresist (not shown) disposed upon its front surface. The wafer "W" isexposed to actinic (relative to the resist) X-ray radiation 240, whichfirst passes through the image mask. Preferably, the distance "d" isbetween 0.5 and 3.0 μm, so that the image mask is sufficiently close tothe front surface of the wafer to cause a pattern formed in the maskingmaterial 230 to be imaged onto the photosensitive material on the frontsurface of the wafer "W". At greater distances from the wafer, theimaging ability of the image mask would suffer, unless the irradiatingenergy were perfectly collimated. In contrast to the present invention,for photolithography using light as the irradiating source, employing ataking lens (see 124, FIG. 1) is effectively the only practical way offaithfully replicating a pattern from the image mask onto the surface ofthe wafer.

FIG. 2b is a cross-sectional view of an alternate embodiment of an X-raymask assembly 200b comprising an image mask 215b and carrier 210,according to the invention. A patterned opaque layer 230 of an X-raymasking material, as described hereinabove, is disposed on the"downstream" surface (right hand surface, as shown in the Figure) of aplanar beryllium substrate 220 to form the mask 215b. As in theembodiment of FIG. 2a, the mask 215b is fastened to the carrier 210. Inthis case, however, this patterned opaque layer 230 is on the wafer side(downstream side) of the mask 215b. As described hereinabove withrespect to FIG. 2a, the carrier 210 preferably has a coefficient ofexpansion similar to that of the beryllium substrate 220.

As with the previous embodiment (of FIG. 2a), the image mask 215b ispositioned a distance "d" upstream of the front surface of a sensitizedsemiconductor wafer "W" such that the plane of the mask is parallel tothe front (planar) surface of the wafer. The sensitized wafer "W" has alayer of X-ray sensitive resist (not shown) disposed upon its frontsurface. The wafer "W" is exposed to actinic (relative to the resist)X-ray radiation 240 through the mask. Preferably, the distance "d" isbetween 0.5 and 3.0 μm, so that the pattern formed by the maskingmaterial 230 is faithfully reproduced into the sensitized layer (e.g.,of photoresist) on the semiconductor wafer "W".

The near-field, afocal, imaging (shadow imaging) technique describedhereinabove with respect to FIGS. 2a and 2b is analogous to photographiccontact printing, in that the mask (analogous to a negative) is placedalmost directly on the wafer (analogous to photographic paper) tofaithfully reproduce an image from the image mask onto the wafer,without using optics (i.e, without focusing). In the case ofsemiconductor device fabrication, the image on the wafer creates apattern in a photosensitive layer on the wafer, which through subsequentremoval of all but the image portions of the photosensitive layer isused to create (e.g., by etching) patterns (e.g., lines) in underlyinglayers (not shown) on the wafer (such as an underlying layer ofpolysilicon). The term "photosensitive layer" is used herein to mean alayer of material that chemically reacts (converts) in the presence ofactinic (chemical conversion causing) radiation, such as X-rays.

Although the use of X-rays for semiconductor lithography is advantageousin terms of its inherent higher resolving ability (i.e., higher thanlight), inter alia, high-quality, fluent X-ray sources tend to be veryexpensive and consume a great deal of power. Hence, according to thepresent invention, gamma-rays can be employed (rather than X-rays) asilluminating sources for semiconductor lithography. As mentioned above,gamma-rays are even shorter in wavelength than X-rays. Hence, gamma-raysare even less resolution-limited than X-rays. Materials are availablethat are relatively transparent to gamma rays, and materials which aresubstantially opaque with respect to gamma rays. It is contemplated bythis invention that these materials could be substituted for thematerials described above, for making a gamma-ray based image mask fornear field lithography (using a gamma ray source for illuminationinstead of an X-ray source). The structures and methods described hereinwith respect to FIGS. 2a and 2b, with such different materials, would beusable and are contemplated for gamma-ray, instead of X-ray lithography.In the main hereinbelow, the use of gamma-rays (radiation) for directwrite lithography, rather than near field lithography is discussed.Generally, the use gamma-rays for lithography of integrated circuits hascertain significant advantages, including:

1. extremely high source brightness (intensity), due to the extremeintensity of naturally occurring gamma-ray sources such as Cobalt-60,which is a passive source requiring no power;

2. high inherent resolution, due to short wavelength;

3. large depth of field, due to short wavelength;

4. resist materials can be fabricated (as discussed in greater detailhereinbelow) which exhibit a high cross section (absorptivity) withrespect to gamma-rays, and the resist materials can be applied to asemiconductor wafer;

5. materials are available which are not only highly opaque to gammarays, but which also emit secondary emissions (primarily in the form ofphotons) which can be used with conventional photoresist materials (asdiscussed in greater detail hereinbelow);

6. numerous chemistries are available which can be exploited to creategamma-ray sensitive resist materials (discussed in greater detailhereinbelow);

7. mechanisms are available (according to the present invention) forbeam modification (as discussed in greater detail hereinbelow) andshuttering (as discussed in greater detail hereinbelow.

The use of gamma-rays as an exposure (illumination) source forintegrated circuit lithography depends, of course, on the use ofsuitable materials for the resist process. In lithographic processes, aresist layer on the surface of a semiconductor wafer is exposed througha mask to an actinic radiation (illumination) source. The resistresponds, in the areas where it is exposed (illuminated), by chemicallyconverting. The unconverted areas are then chemically removed. (This isa "negative" process. "Positive" processes also exist whereby theexposed areas are chemically removed. Both positive and negative typeprocesses are contemplated.) The result is a patterned etch resistantlayer.

Conventional photolithography uses one of a number of organic resistmaterials (e.g., polystyrene, phenolformaldehyde, polyurethane, etc.).These materials are photo-sensitive (convert chemically when exposed tovisible light radiation) and have good etch resist characteristics forsubsequent wafer etching. These conventional organic resist materialsare suitable for gamma-ray lithography in all respects except that theirintrinsic absorbance of gamma-rays is very low.

As is well known in the art, silicon integrated circuits, including CMOScircuits, have a finite tolerance to gamma-ray exposure. Excessiveexposure to gamma-rays can cause field inversion in MOS FETs(Metal-Oxide-Semiconductor Field Effect Transistors) leading toexcessive current leakage or outright device failure, and various otherserious problems.

It is necessary, therefore, to provide a gamma-sensitive resist materialwhich either: a) requires very short exposure times, or b) provides asufficiently high cross-section to gamma-rays (absorbs gamma-rays wellenough) so that the underlying wafer is effectively "shielded" from thegamma radiation by the resist material itself.

According to the present invention, it is recognized that gammaradiation, being an ionizing radiation, is capable of causing secondaryemissions in various materials. For example, when tungsten is exposed togamma radiation, gamma radiation is effectively completely absorbed bythe tungsten, and the energy of the gamma radiation causes electrons tobe "ripped" from their ordinary orbits (shells) in the atomic structureof the tungsten, thereby causing the tungsten to become ionized. In theprocess, the change of energy levels causes the ionized tungsten to emitphotons of energy at a different wavelength (secondary emissions). Aswill be evident from the discussion hereinbelow, these secondaryemissions are compatible with (actinic with respect to) essentiallyconventional photoresist materials.

Tungsten, having a high inherent cross-section (absorptivity) withrespect to gamma radiation, i.e., gamma radiation does not pass throughit particularly well, makes an effective gamma radiation shield whenpresent over the surface of a semiconductor device.

For example, by applying a layer of tungsten over a conventional organicresist layer (on the upstream side of the resist), the secondaryemission properties of the tungsten in response to gamma radiation maybe utilized to "expose" the resist, while simultaneously shielding theunderlying wafer from gamma radiation. The organic resist materials havea high cross-section to the secondary radiation. Various combinations ofconventional photoresist materials with tungsten added into or onto thephotoresist are disclosed hereinbelow (with respect to FIGS. 3a-3d), andfor purposes of this discussion are termed "compound resists". They areall effectively converted when exposed to incident gamma radiation, andcan all be formulated to effectively shield the underlying semiconductordevice (if necessary).

FIG. 3a is a cross-sectional view of a sensitized semiconductor wafer300a illustrating an embodiment of a gamma-sensitive compound resist. Afront surface "S" of a semiconductor wafer 310 is coated with a layer320a of photoresist, for example conventional organic photoresistmaterial such as polystyrene, phenolformaldehyde, polyurethane, etc.Preferably, the layer of photoresist is applied as a planar layer, inany suitable manner.

A layer 330a of a material with a high cross-section to gamma radiation,such as tungsten, boron or bromine, is disposed over the resist layer320a. This may be referred to as a "secondary resist layer"--togetherthe layers 320a and 330a forming a "compound resist" sensitizing thefront surface "S" of the wafer 310 to incident gamma radiation.Preferably, the secondary resist layer 330a is formed as a film oftungsten, and is preferably of sufficient thickness to absorb allincident gamma radiation. However, the layer 330a must also be fairlythin, to allow secondary emissions to enter the underlying resist layer320a.

Gamma radiation 340, impinging upon the secondary-resist layer 330aionizes the material of the secondary resist layer 330a, causingscattered secondary photons emissions 345a having a different (generallylonger) wavelength than that of the gamma radiation to be emitted intothe resist layer 320a. The secondary emissions 345a are preferably ofsubstantially shorter wavelength than visible light, and are employed toconvert the underlying resist material 320a. In this manner, thelithography technique of the present invention is not as resolutionbound as conventional photolithography. Preferably, the secondaryemissions 345a are substantially shorter in wavelength than visiblelight, and are nevertheless capable of converting (acting actinicallywith respect to) the underlying photoresist 320a.

The resist material 320a is highly absorbent of the secondary emissions345a and thereby limits the exposure of the resist layer 320a to thesecondary emissions 345a to a small area 360a about the point where thegamma radiation 340 strikes the secondary resist layer 330a. Afterchemically (or mechanically) stripping the secondary resist layer 330a,the unconverted areas of the resist layer 320a are removed, leaving an"island" of etch-resistant resist material over an area 350a of thesemiconductor wafer 310. This describes forming a point feature on thewafer. A layer to be patterned, such as a layer of polysiliconunderlying the resist is omitted for illustrative clarity (in all ofFIGS. 3a-3d). With a finely collimated gamma beam in fixed position, thewafer 310 can be "walked around" so that the beam 340 can describe andconvert a line of photoresist. This would be a so-called "direct write"technique for semiconductor lithography.

In the case that the gamma radiation were to impinge on the sensitizedwafer 300a through a mask (not shown), two dimensional patterns could beformed directly on the photoresist 320a. This would be "near-field"semiconductor lithography (compare FIGS. 2a and 2b).

As shown in FIG. 3a, the secondary emissions 345a tend to scatter, inother words be emitted in directions at an angle to the incident beam340. This causes a limited amount of "blooming" (or de-focusing of apattern through an image mask). However, the intensity and direction ofthe gamma radiation 340 cause the bulk of the secondary emissions 345ato be emitted substantially in the direction of travel of the incidentgamma beam 340. Further, high absorbency of the resist layer relative tothe secondary emissions limits the amount of blooming. Further, sincethe secondary emissions do not travel any significant distance, theirdivergence from the path of the incident gamma beam is relativelyinsignificant (they do not have an opportunity to go in the "wrong"direction for very far). By controlling the beam diameter (in directwrite applications), or by adjusting the mask pattern (in near fieldapplications), to compensate for any blooming, it is possible toaccurately control the feature size (area 350a). Vis-a-vis direct writelithography techniques, evidently the photoresist can be patterned witha fineness--having a critical dimension (cd)--substantially approachingthe diameter of the beam. Techniques for creating an extremely smalldiameter, collimated beam of gamma radiation are discussed hereinbelow.

Another approach to making a gamma-sensitive resist from conventionalorganic resist materials is to make use of the same secondary emissionproperty of a secondary material in a slightly different way. Thecompound gamma-resist material shown and described with respect to FIG.3a was formed by depositing an overlying, upstream layer of a secondaryemitter (330a) with a high cross-section to gamma radiation over theorganic resist (320a). If instead the organic resist material is dopedwith the secondary emitter, a homogenous, gamma-sensitive, compoundresist can be formed.

FIG. 3b is a cross-sectional view of a sensitized semiconductor wafer300b employing a doped type of gamma-sensitive compound resist. As inFIG. 3a, an underlying layer of material to form semiconductor features(such as a layer of polysilicon) is omitted for illustrative clarity.Beginning with a base of substantially conventional, preferably organicphotoresist material 320b, the photoresist 320b is doped with particlesof a material which will absorb gamma rays and emit secondary emissions.A representative particle 325b is illustrated, and functions in a mannersimilar to the layer 345a of FIG. 3a.

Preferably, the doped, base resist layer 320b is a conventional organicresist material, such as polystyrene, phenolformaldehyde, polyurethane,etc., and it is doped with a secondary emitting dopant (e.g., dopantparticle 325b) with a high cross-section to gamma radiation, such astungsten, boron, or bromine. Gamma radiation 340 is shown impinging upona representative secondary emitter dopant particle 325b, causing it tobecome ionized, resulting in scattered secondary photon emissions 345bhaving a different wavelength than that of the gamma radiation. The"base" (undoped) resist material is highly absorbent of the secondaryemissions 345b and thereby limits the exposure of the resist layer 320bto the secondary emissions 345b to a small area 360b about the pointwhere the gamma radiation 340 strikes the particle 325b. After exposure,the unconverted areas of the doped resist layer 320b are removed,leaving areas (points or lines) of converted compound resist over anunderlying layer (e.g., polysilicon) on the front surface "S" of thewafer 310. The number of particles (e.g., 325b) required to be "mixed"into the base photoresist is determined by the intensity of the incidentgamma radiation. The particles (e.g., 325b) may be uniformly distributedthroughout the base photoresist, for example by mixing the particlesinto the photoresist material prior to applying the photoresist materialto the surface of the wafer. On the other hand, the particles can be"implanted" into the surface of photoresist already applied to thesurface of the wafer, in which case there will be a concentrationgradient of particles more concentrated towards the surface of thephotoresist (away from the wafer). Other gradients or non-uniformconcentrations of particles are also contemplated.

Although the base photoresist is most sensitive to the secondaryemissions (345a, 345b), it bears mention that the base photoresist mayalso be somewhat sensitive to the direct effects of gamma-rayirradiation. However, it is preferred that the process parameters beadjusted so that little or no gamma radiation reachs the underlyingsemiconductor wafer 310. Hence, the dopant concentration (or thicknessof the film 330a, in FIG. 3a), as well as the transparency of thephotoresist with respect to gamma radiation, as well as the sensitivityof any underlying structures to gamma radiation must be taken intoaccount when performing the semiconductor lithography techniques of thepresent invention.

Again assuming that the gamma radiation reaches the sensitized wafer300b through a mask (not shown), forming patterns of intense gammaradiation on the surface of the doped resist layer 320b, the scatteringof the secondary emissions 345b causes a certain amount of "blooming" orde-focusing of the pattern, for the reasons described hereinabove. Asbefore, the intensity and direction of the gamma radiation 340 cause thebulk of the secondary emissions 345b to be emitted substantially in thedirection of travel of the gamma radiations. Even though the secondaryemitter (particle 325b) is disposed within the organic resist as adopant, it still has a high cross-section to gamma radiation andeffectively shields the semiconductor wafer 310 from excessive exposure.

Both of these embodiments of gamma-sensitive resist, i.e., the two-layercompound resist described with respect to FIG. 3a and the doped compoundresist described with respect to FIG. 3b, provide the desiredcharacteristics of sensitivity to gamma radiation and inherent gamma-rayshielding, thereby acting as an effective resist while preventingexcessive exposure of the underlying semiconductor wafer to gamma rays.

In the two-layer gamma-sensitive resist embodiment shown and describedwith respect to FIG. 3a, the secondary resist layer (330a) is notgenerally chemically sensitive to the incident radiation (340). Itsimply serves as a secondary emitter which serves to simultaneouslyblock the incident gamma-radiation and to convert the incident radiationto another type of radiation which is actinic with respect to a resistlayer (or base resist) and of which the underlying integrated circuitry(on wafer 310) is more tolerant. There are materials, however, suitablefor use as a secondary emitter which are themselves chemically sensitiveto exposure to gamma radiation. Further, some organic (and inorganic)resist materials are at least somewhat chemically sensitive to exposureto gamma radiation. Accordingly, it is possible to form multilayergamma-sensitive resist coatings where the top layer (overlayer) providessecondary emissions and is also chemically converted by exposure togamma-radiation. If the bottom layer (underlayer, between the overlayerand the wafer) is gamma-sensitive, then the use of an overlayer whichdoes not completely block gamma radiation permits exposure of theunderlayer by both direct (leaked through the overlayer) gamma radiationand by secondary emissions in the overlayer.

Several benefits are derived from the use of multilayer gamma-sensitiveresist. First, the use of dual chemistries in combination permitsconsiderably greater flexibility and versatility in determining overallresist characteristics. Second, a multilayer resist tends to permitbetter planarization of the resist surface. (Planar layers insemiconductor devices are generally sought-after objectives.) It is wellknown in the art that a truly planar surface is more easily obtained intwo steps (i.e., an extremely planar surface is easier to form on top ofa surface which is already substantially planar) than in one step.Improved surface planarity of a resist coating tends to enhance thelinewidth uniformity of patterns created by incident radiation.(Linewidth uniformity in semiconductor layers is a generallysought-after objective.)

FIGS. 3c and 3d illustrate two embodiments of multilayer,gamma-sensitive, compound resists, according to the present invention.

FIG. 3c is a cross-sectional view of a sensitized semiconductor wafer300c employing a multi-layer gamma-sensitive resist (320c/330c). A frontsurface "S" of the semiconductor wafer 310 is coated with a primaryresist layer 320c, which is sensitive to both gamma radiation andsecondary emissions. On top of this primary layer 320c is disposed asecondary gamma-sensitive resist layer 330c of a material with arelatively high cross-section to gamma radiation, but which permits somegamma radiation to pass through it. Gamma radiation 340 impinging uponthe secondary resist layer 330c ionizes the material of the secondaryresist layer 330c, causing scattered secondary (photon) emissions 345cwhich enter the primary resist layer 320c with, generally, a differentwavelength than that of the gamma radiation being emitted into thesecondary resist layer 330c. A portion 340' of the incident gammaradiation 340 (indicated by dashed line and arrow) passes through thesecondary resist 330c and into the primary resist 320c. The primaryresist material is highly reactive to the secondary emissions 345c andthereby limits the exposure of the primary resist layer 320c to thesecondary emissions 345c to a small area 360c about the point where thegamma radiation 340 strikes the secondary resist layer 330c. The primaryresist 320c is also chemically sensitive to the "leaked" gamma radiation340' (gamma radiation is actinic to the primary resist), a factor whichenhances the chemical conversion of the primary resist 320c, improvingcontrast. After chemically "developing and stripping the unconvertedareas of the primary and secondary resist layers 320c and 330c,respectively, an "island" of etch resist remains over an area 350c ofthe semiconductor wafer 310 as shown. Complete patterns may be formed onthe resist layers, in the manner described above, and the resist patternmay be transferred to an underlying layer (not shown) as describedabove.

FIG. 3d shows another arrangement of a sensitized semiconductor wafer300d employing a multilayer gamma-sensitive resist. Again, a frontsurface "S" of semiconductor wafer 310 is coated with a primary resistlayer 320d. Preferably, the resist layer 320d is a conventional organicresist material, such as polystyrene, phenolformaldehyde, polyurethane,etc. A secondary gamma-sensitive resist layer 330d of a gamma-sensitivematerial with a high cross-section to gamma radiation is disposed overthe resist layer 320d. Gamma radiation 340, impinging upon the secondaryresist layer 330d simultaneously chemically convents and ionizes an areaof the material of the secondary resist layer 330d, causing scatteredsecondary (photon) emissions 345d, generally having a differentwavelength than that of the gamma radiation 340 being emitted into thesecondary resist layer 330d. The secondary resist material 330d ishighly absorbent of the secondary emissions 345d and thereby limits theexposure of the primary resist layer 320d to the secondary emissions345d to a small area 360d about the point where the gamma radiation 340strikes the secondary resist layer 330d. The high cross-section of thesecondary resist layer 330d to gamma radiation prevents leakage of gammaradiation 340 through the secondary resist layer 330d into the primaryresist layer 320d and the underlying wafer 310, thereby limiting theexposure of the wafer 310 to gamma radiation. After chemicallydeveloping and stripping the unconverted areas of the primary andsecondary resist layers 320d and 330d, an "island" of etch resistremains over an area 350d of the semiconductor wafer 310 as shown.Patterning and processing is performed as described above.

The compound resists described above With respect to FIGS. 3a-3d areuseful for either near field or direct write lithography, both of whichprocesses are afocal. Near field lithography utilizes an image mask inclose proximity to the sensitized surface of the wafer, as describedwith respect to FIGS. 2a and 2b, and is preferably performed with X-rayradiation. Direct Write lithography is preferably performed withgamma-rays, and requires a tightly focused or collimated beam of radiantenergy directed to specific locations of a resist layer, therebyexposing and chemically converting those areas and forming patterns forprocessing lines and the like in layers underlying the resist layer.However, both X-ray and gamma-ray radiation may be used for eitherdirect-write or near-field lithography, such combinations beingcontemplated herein as within the scope of the present invention.

Because of the high inherent resolution capability of gamma radiation(short wavelength), the prospect of direct-write gamma lithography isvery attractive. In order to accomplish this, however, it is necessaryto provide, in addition to the gamma-sensitive resists describedhereinabove, means for generating a tightly focused or collimated beamof gamma radiation, and means for "shuttering" or gating the beam.Suitable means for collimating and shuttering are described hereinbelowwith respect to FIGS. 4 and 5a-c, respectively.

According to the invention, a broad incident beam of radiation (or aradiant point source) can be concentrated and collimated, providing avery narrow, intense beam of radiation useful over a range of distances.This is accomplished by using a hollow, horn-shaped (or conical) afocalconcentrator of the type schematically depicted in FIG. 4.

FIG. 4 is a diagrammatic view of an afocal concentrator 400 forproviding a very narrow, collimated beam of radiation. The afocalconcentrator 400 has a tapered input (upstream) section 410 and anoptional cylindrical output (downstream) section 450. The taperedsection has a broad upstream mouth 420 (analogous to the bell of atrumpet) and a narrow opening 425 at an opposite downstream end thereof.The cylindrical section 450 has a diameter "do" equal to the diameter ofthe narrow opening 425, and is preferably formed contiguously therewith(i.e., the tapered and cylindrical sections are preferably formed as aunit structure). A relatively broad incident beam of radiation (e.g.,gamma radiation) enters the mouth 420 of the tapered portion 410. (Sucha beam could be generated by any suitable means including by a chemicalradiant source, with or without a backing reflector.) The radiation beamis indicated by representative rays 440a and 440b entering oppositeouter peripheral (circumferential) portions of the mouth 420. In thetapered section 410 as shown, the taper is approximately exponential,however any tapered form (e.g., a linear taper forming a conical shape),may be employed. An inner surface 415 of the afocal concentrator 400 isreflective of the incident radiation, and serves to reflect andconcentrate the incident radiation towards the narrow opening 425 of thetapered section 410. For example, the tapered section could be formed ofaluminum, nickel or chromium, or plated with the same on its innersurface (bore), to reflect and concentrate gamma rays. The cylindricalsection 450, which should also have a highly reflective bore, serves tofurther collimate this concentrated radiation beam, providing anintense, narrow, collimated output beam 460 at an output end 455thereof.

In practice, the output beam 460 is not perfectly collimated and willdiverge to some degree. However, over a first distance, d1, the outputbeam 460 remains roughly converged to within approximately the diameter`do` of the output end 455 of the cylindrical section 450 (or of thenarrow opening 425 if the cylindrical section 450 is not Used). Assuminga maximum useful (for direct write lithography) beam diameter "dm", theoutput beam 460 is useful over a distance of up to "d2" (d2>d1, as shownhere) from the output end 450 of the cylindrical section (or of thenarrow opening 425 if the cylindrical section 450 is not used). Thelonger the afocal concentrator 400, especially the longer the taperedportion of the concentrator, generally the better the collimation of theoutput beam 460 can be (i.e., long taper=less beam divergence). Thedimension "d2" represents the useful effective (for direct writelithography) depth of field of the concentrator 400.

In practice, for direct write semiconductor lithography, a radiantsource, such as a pellet of Cobalt-60, is placed as close to the mouthof the concentrator as possible. As mentioned hereinabove, the emissionsfrom the source can be directed more-or-less exclusively towards themouth of the concentrator 400 by providing a reflector (compare 114,FIG. 1) upstream of the source (compare 112, FIG. 1). The resultantoutput beam 460 exiting the concentrator 400 is intense, adequatelycollimated, highly fluent (given a fluent source such as Cobalt-60), andvery highly homogenous (minimal or negligible hot spots in the crosssection of the beam 460 due to the many reflections experienced by thebeam in the concentrator 400).

Exemplary dimensions for the concentrator, for semiconductor lithographyare: Preferably, the mouth of the concentrator is between 50 μm and 60μm in diameter, and the output diameter "do" is less than 0.5 μm indiameter. The output diameter "do" can be made as small as desired, forexample 0.1 μm, for converting extremely small areas of resist materialon a semiconductor wafer (compare 360a-d in FIGS. 3a-d). Evidently, toform converted lines in the resist material, in a direct-writeapplication, one or the other of the concentrator 400 or thesemiconductor wafer must "walk around" in the plane of the wafer surface(or the appropriate differential angle may be utilized to target areaswithout perpendicular beam impingement). Given the relative complexitiesof walking around the concentrator or the wafer, it is preferred thatthe concentrator remain stationary and that the wafer be moved around ina plane (X-Y positioning). High resolution positioning platforms areavailable for "walking" the wafer around. Given a "passive" gamma sourcesuch as Cobalt-60, it is evident that a mechanism must be provided forgating (turning on and off) the output beam (460). Else, walking aroundthe wafer would produce an endless line. A shutter mechanism for gatingthe output beam is described below with respect to FIGS. 5a-c.

Although the rays 440a and 440b are shown in FIG. 4 as parallel raysentering the "bell" (mouth 420) of the concentrator 400, the rays of theincident beam need not be parallel (collimated). The concentrator willcollimate input rays that are not parallel. However, the less parallelthe input rays, the more collimation must be performed by thecollimator. These factors need to be taken into account in the design ofan overall lithography system. The better the initial collimation of theincident beam, the better the ultimate collimation of the output beam.One way to improve the input collimation is to position the source ofthe incident beam distant from the mouth of the concentrator (relativeto the size of the concentrator. This serves to make the rays of theincident beam that actually enter the mouth of the concentrator moreparallel with one another, thereby improving output collimation. If thesource is positioned far away from the mouth of the collimator, it isalso possible to use a cylindrical pipe (not shown) between the sourceand the mouth of the concentrator to help collect and direct radiationfrom the source. For example, a Cobalt-60 pellet could be placed in aclosed end of a cylindrical tube, the closed end serving as=an upstreamreflector. The tube would be oriented coaxial to the concentrator, withits downstream open end placed adjacent the mouth of the concentrator.The inside surface of the tube would be highly reflective. In thismanner, emissions of gamma radiation into the environment (other thantowards the wafer) could be minimized.

Although the instant application of the afocal concentrator 400 is toconcentrate a gamma radiation beam, the same technique is applicable toany form of radiation source, including X-rays, UV light, and visiblelight. A major difference between such afocal concentrators fordifferent radiation sources would be the material of which the innersurface of the concentrator (e.g., 415) is formed (or plated). The mainrequirement for the material of the inner surface of the concentrator isthat it be reflective of the range of wavelengths in the incident beam.Aluminum is reflective of many different radiation wavelengths,including gamma radiation, and is suitable for gamma lithography. Nickeland Chromium are also suitably reflective materials. It is within thespirit and scope of the present invention that the afocal concentratordescribed hereinabove be applied to concentrate any suitable radiationsource. The bore size of the concentrator also depends on the desiredsize of the output beam. Extremely small bore diameters can be formed byetching, ion milling, and other "machining" techniques which are known.Although the concentrator may be somewhat expensive to manufacture withprecision, its cast will readily be amortized over the course offabrication for a great number of semiconductor devices. And, asmentioned hereinabove, a passive source, such as Cobalt-60, provides agreat deal of energy without consuming any external power.

A shutter mechanism for gating the output beam from the concentrator isdescribed hereinbelow with respect to FIGS. 5a-c.

According to the invention, a surface acoustic wave (SAW) deviceoperating as a shallow angle reflecting/scattering surface, can operateas a shutter element for X-ray or gamma-ray (or other) radiations. Athin, reflective film of, for example, aluminum, nickel, or chromium, isdisposed over the surface of a Surface Acoustic Wave (SAW) device.Assuming that the SAW device is not activated, the reflective surface issubstantially planar, and reflects any incident energy (e.g., a beam ofgamma rays) at an angle equal and opposite to its angle of incidence. Atightly collimated beam approaching at a known shallow angle, will bereflected off of the reflective surface of the unactivated SAW device ata predictable shallow angle. If the SAW device is activated, however,the surface of the SAW device becomes distorted and deflects or scattersthe incident beam. By providing a beam stop or an aperture, andpositioning the beam stop or aperture such that radiation from theincident beam will pass the beam stop or aperture only whenshallow-angle-reflected off of the surface of the SAW device, aneffective shutter mechanism can be implemented.

FIGS. 5a-c illustrate the operation of this Surface Acoustic Waveshutter device, as exemplary of a shallow-angle-reflection,distortable-surface shutter mechanism. SAW devices are generally knownfor other (than the shallow angle shutter disclosed herein) purposes,such as for imposing propagation delays on travelling waves, allowingparticular wavefronts to be selectively "picked off" from the end of theSAW device.

FIG. 5a is a side view of a shutter mechanism 500 employing a SAW devicein its unactivated (planar, non-deformed surface) state. The shutter isformed of a Surface Acoustic Wave device 510 with a planar top surfaceupon which a reflective film 510a is disposed, and a strategicallypositioned beam-stop (or "knife edge") 520. A collimated (directional)incident beam 540 approaches the reflective surface 510a of the SurfaceAcoustic Wave device 510 at a shallow angle, and is reflected by thereflective surface 510a at an equal and opposite angle, forming areflected beam 540a. The trajectory of the reflected beam is such thatit misses the beam stop 520 and continues traveling along the sametrajectory.

FIG. 5b is a side view of the shutter mechanism 500 of FIG. 5a, with theSurface Acoustic Wave device in an activated (surface-deformed) state.Electrical stimulation of the Surface Acoustic Wave device causessurface Waves 530 or distortions (shown greatly exaggerated) to beformed on the reflective surface 510b. For the same shallow angleincident beam 540 (compare FIG. 5a), these surface distortions 530 causethe reflected beam 540b to be scattered or diverted relative to theposition of the reflected beam 540a from the unactivated SurfaceAcoustic Wave device 510. In other words, the beam 540b is reflected ata different angle off the distorted surface than the beam 540a isreflected from the undistorted surface of the SAW device. As a result,the reflected beam strikes the beam stop 520 and is blocked thereby suchthat the reflected beam does not exit the Surface Acoustic Wave shutter500. By selectively energizing (activating) the SAW device, the beam 540is effectively gated (turned on and off). This allows discrete lines(e.g., of converted resist material) to be formed on the surface of asemiconductor device.

FIG. 5c is a greatly enlarged (magnified) view of a portion of theSurface Acoustic Wave shutter device 510, in the surface-distorted stateshown in FIG. 5b, showing the point of reflection. As before, thedistortions are shown greatly exaggerated. A reference line 550indicates the location and angle of the undistorted surface (see, e.g.,510a). A tangent line 560 indicates the angle of the reflective surface510b at the point of reflection. The incident beam 540, approaches thereflective surface 510b of the Surface Acoustic Wave device 510 at ashallow incident angle Θih relative to the horizontal (i.e., relative tothe reference line 550). However, due to the distortion of thereflective surface 510b, the effective angle of incidence relative tothe tangent line 560 is a steeper angle, shown as Θi, where Θi>Θih (asshown). As a result, the incident beam 540 is reflected as a reflectedbeam 540b at a reflection angle of Θr=Θi relative to the tangent line560. The effective reflection angle Θrh of the reflected beam 540brelative to the reference line 550 (horizontal plane) is even greater(as shown). By this mechanism, the incident beam 540 can be reflectedsuch that it is either blocked or passed by a beam stop or apertureunder electrical control. Note that the beam stop 520 effectively formsan "aperture" with the surface 510a of the Surface Acoustic Wave device510. Alternatively, an aperture may be provided instead of a"knife-edge" style of beam stop 520.

It is not necessary, according to the invention that the incident beam540 be "cleanly" reflected in any particular direction. It is onlynecessary that the reflected beam 540b be reflected anywhere other thanpast the beam stop or aperture 520.

It will readily be apparent to one of ordinary skill in the art that amagnetostrictive device may be substituted for the Surface Acoustic Wavedevice 510 to accomplish a similar result (directing and diverting abeam of incident radiation). Both magnetostrictive and Surface AcousticWave devices act as a sort of "surface distortion device" for thepurposes of the present invention. Any device that can reflect anincident beam at one angle in one energized state (e.g., not energized),and reflect an incident beam at another angle in another energized state(e.g., energized), in conjunction with a beam stop or aperture allowinga reflected beam to pass only at a critical angle, can be employed inthe instant inventive shutter mechanism. An advantage of using a surfacedistortion device, rather than a device which must physically bepositioned, is that the response time of such surface distortion devicesis relatively quick. This enables such a device, in conjunction with abeam stop or aperture, to be used as a high-speed shutter (e.g., nomoving parts).

It will also be readily apparent to one of ordinary skill in the artthat this type of shutter device may be applied to radiation of avariety of wavelengths, including gamma-rays, X-rays, UV light, etc. Itis within the spirit and scope of the present invention that the SAW (ormagnetostrictive) shutter device described hereinabove be applied as ahigh-speed shutter to any suitable form of radiation beam.

FIGS. 6a and 6b are block diagrams of (gamma-ray) direct-writelithographic apparatus employing the techniques described hereinabovewith respect to FIGS. 3a-d, 4, and 5a-c; or for a near-fieldlithographic apparatus employing the techniques described hereinabovewith respect to FIGS. 2a-b, 3a-d 4 and 5a-c.

FIG. 6a is a block diagram of a direct-write (or energy source fornear-field; gamma-ray, x-ray or other radiation) lithography(lithographic) apparatus 600a, according to the present invention. Aradiation source 610 provides a source of intense directional gamma-ray(or x-ray, or other radiation, collectively herein called "gamma-ray")radiation. A suitable passive gamma radiation source is Cobalt-60 whichpassively radiates intense gamma-ray radiation. A reflector (such asthat shown and described as 114 with respect to FIG. 1) may be employedto improve the directionality and intensity of the source 610. Given anyshutter 630, or other "on/off" mechanism, the beam 640 need not be verywell collimated. Given a shutter 630 such as was described with respectto FIGS. 5a-5c, a collimator similar to that shown in FIG. 4 could beemployed to direct the beam 640 into the shutter 630.

Incident gamma-ray radiation 640 from the gamma-ray radiation source 610enters a shutter device 630, such as the Surface Acoustic Wave shutterdevice shown and described as 500 with respect to FIGS. 5a-5c. Theshutter device 630 serves to selectively gate (block or pass) theincident beam 640, resulting in a controlled gamma-ray beam 640a. Thecontrolled gamma-ray beam 640a enters the mouth of an afocalconcentrator 620, such as that shown and described (400) with respect toFIG. 4. The afocal concentrator narrows and intensifies the controlledbeams 640a to provide a collimated beam 640b. A semiconductor wafer 650is positioned a distance "d3" from the output of the afocal concentrator620 such that the collimated beam impinges upon the front surface 655thereof. The front surface 655 of the wafer 650 is coated with a layer660 of gamma-sensitive resist, such as that shown and described as320a-d (and optionally 330a, c-d) with respect to FIGS. 3a-3d,respectively. Preferably, the wafer is mounted to a movable carriage(not shown) for direct-write application, by which means the wafer 650may be positioned such that the collimated beam 640b may be caused toimpinge on any point on the resist layer 660. The on/off state of thecollimated beam 640b may be effectively controlled by selectivelyactivating and de-activating the shutter device 630. Preferably, thedistance "d3" is approximately 5 μm. Preferably, the distance "d3"should be no greater than the distance "d2" shown in FIG. 4.

FIG. 6b is a block diagram of an alternate embodiment of a direct-writegamma-ray (or x-ray or similar radiation, collectively herein called"gamma-ray") lithographic apparatus 600b, according to the invention. Asbefore, a radiation source 610, such us Cobalt-60, provides a source ofintense directional gamma-ray radiation. A reflector (such as that shownand described as 114 with respect to FIG. 1) may be employed to improvethe directionality and intensity of the source 610. Incident gamma-rayradiation 640 from the gamma-ray radiation source 610 enters the mouthof an afocal concentrator 620, such as that shown and described (400)with respect to FIG. 4. The afocal concentrator narrows and intensifiesthe incident gamma-ray radiation 640 to provide a collimated gamma-raybeam 640a'. The collimated gamma-ray beam 640a' enters a shutter device630, such as the Surface Acoustic Wave shutter device shown anddescribed as 500 with respect to FIGS. 5a-5c. The shutter device servesto selectively gate (block or pass) the collimated gamma-ray beam 640a',resulting in a collimated, controlled gamma-ray beam 640b'. Asemiconductor wafer 650 is positioned a distance "d3" from the output ofthe afocal concentrator 620 such that the controlled collimated beam640b' impinges upon the front surface thereof. The front surface 655 ofthe wafer 650 is coated with a layer 660 of gamma-sensitive resist, suchas that shown and described as 320a-d (and optionally 330 a, c-d) withrespect to FIGS. 3a-3d, respectively. Preferably, the wafer 650 ismounted to a movable carriage (not shown) for direct-write application,by which means the wafer 650 may be positioned such that the collimatedcontrolled beam 640b' may be caused to impinge on any point on theresist layer 660. The on/off state of the collimated controlled beam640b' may be effectively controlled by selectively activating andde-activating the shutter device 630. In this configuration, the shutterdevice 630 is between the output of the concentrator 620 and the wafer650. Hence, the shutter 630 must be made small. Small SAW (ormagnetostrictive) devices can be fabricated to meet this criteria. It isalso possible (in any of the examples set forth herein) that the beammay be reflected off a suitable reflecting surface (not shown) so thatit initially approaches the wafer 650 at an angle (e.g., parallel, orbetween parallel and normal) to the surface of the wafer) and isreflected by the reflector to ultimately impact the wafer at ninetydegrees (normal) to the surface of the wafer.

It is within the spirit and scope of the present invention that theinventive techniques described hereinabove be used either alone or incombination. By employing these techniques, viable forms ofshort-wavelength (e.g., gamma-ray or X-ray) afocal lithography may berealized. It should also be recognized that many of the techniquesdescribed hereinabove may be applied to other types of radiation, suchas UV light or visible light.

We claim:
 1. A semiconductor lithography apparatus comprising:a) anX-ray source emitting X-ray radiation in a direction downstream along apath; b) a semiconductor wafer disposed downstream of said X-ray sourceacross said path; and c) an image mask assembly comprising:i) aperipheral beryllium carrier disposed along said path between said X-raysource and said wafer and spaced substantially parallel to said wafer;ii) a beryllium substrate having a substantially planar surface andperipherally mounted to said beryllium carrier; iii) a patterned metalmasking material disposed on said substantially planar surface of saidberyllium substrate, said patterned masking material furthercharacterized as:(1) relatively opaque to X-ray radiation, (2) adherentto beryllium, (3) insensitive to moisture, (4) resistant to corrosion,and (5) having a thermal coefficient of expansion substantially equal tosaid beryllium substrate and said beryllium carrier; whereby said imagemask assembly, comprising a beryllium carrier, beryllium mask substrate,and masking material having a thermal coefficient of expansionsubstantially equal to said beryllium carrier and mask substrate, willnot be subject to thermal distortion due to the use of materials havingmatched thermal coefficients of expansion.
 2. The image mask assembly ofclaim 1 wherein said patterned metal masking material is selected fromthe group consisting of gold, tungsten, platinum, barium, lead, iridium,and rhodium.
 3. The image mask assembly of claim 1 wherein said imagemask assembly is spaced closely from said wafer, so that said radiationfrom said X-ray radiation source shadows an image of said patternedmetal masking material onto said wafer.
 4. A semiconductor lithographyapparatus comprising:a) an X-ray source emitting X-ray radiation in adirection downstream along a path; b) a semiconductor wafer disposeddownstream of said X-ray source across said path; and c) an image maskassembly comprising:i) a peripheral beryllium carrier disposed alongsaid path between said X-ray source and said wafer and spacedsubstantially parallel to said wafer; ii) a beryllium substrate having asubstantially planar surface and peripherally mounted to said berylliumcarrier; iii) a patterned metal masking material selected from the groupconsisting of gold, tungsten, platinum, barium, lead, iridium, andrhodium, disposed on said planar surface of said beryllium substrate,said patterned masking material further characterized as:(1) relativelyopaque to X-ray radiation, (2) adherent to beryllium, (3) insensitive tomoisture, (4) resistant to corrosion, and (5) having a thermalcoefficient of expansion substantially equal to said beryllium substrateand said beryllium carrier; whereby said image mask assembly, comprisinga beryllium carrier, beryllium mask substrate, and masking materialhaving a thermal coefficient of expansion substantially equal to saidberyllium carrier and mask substrate, will not be subject to thermaldistortion due to the use of materials having matched thermalcoefficients of expansion.
 5. The semiconductor lithography apparatus asin claim 4, wherein: said means for supporting said image mask orientssaid mask so that said masking material is downstream relative to saidsubstrate.
 6. A method of semiconductor lithography comprising the stepsof:a) providing an X-ray source emitting X-ray radiation in a directiondownstream along a path; b) disposing a semiconductor wafer downstreamof said X-ray source across said path; and c) providing an image maskassembly by:i) providing a peripheral beryllium carrier disposed alongsaid path between said X-ray source and said wafer and spacedsubstantially parallel to said wafer; ii) peripherally mounting to saidberyllium carrier a beryllium substrate having a substantially planarsurface; and iii) disposing, on said planar surface of said berylliumsubstrate a patterned metal masking material, said patterned maskingmaterial further characterized as:(1) relatively opaque to X-rayradiation, (2) adherent to beryllium, (3) insensitive to moisture, (4)resistant to corrosion, and (5) having a thermal coefficient ofexpansion substantially equal to said beryllium substrate and saidberyllium carrier; whereby said image mask assembly, comprising aberyllium carrier, beryllium mask substrate, and masking material havinga thermal coefficient of expansion substantially equal to said berylliumcarrier and mask substrate, will not be subject to thermal distortiondue to the use of materials having matched thermal coefficients ofexpansion.
 7. The method of semiconductor lithography of claim 6 whereinsaid step of disposing, on said planar surface of said berylliumsubstrate, a patterned metal masking material, further comprisesselecting said patterned metal masking material from the groupconsisting of gold, tungsten, platinum, barium, lead, iridium, andrhodium.
 8. The method of semiconductor lithography of claim 6 whereinsaid step of providing a peripheral beryllium carrier disposed alongsaid path between said X-ray source and said wafer further comprisesspacing said carrier closely from said wafer, so that said radiationfrom said X-ray radiation source shadows an image of said patternedmetal masking material onto said wafer.
 9. A method of semiconductorlithography comprising the steps of:a) providing an X-ray sourceemitting X-ray radiation in a direction downstream along a path; b)disposing a semiconductor wafer downstream of said X-ray source acrosssaid path; and c) providing an image mask assembly comprising:i) aperipheral beryllium carrier disposed along said path between said X-raysource and said wafer and spaced substantially parallel to said wafer;ii) a beryllium substrate having a substantially planar surface andperipherally mounted to said beryllium carrier; iii) a patterned metalmasking material selected from the group consisting of gold, tungsten,platinum, barium, lead, iridium, and rhodium, disposed on said planarsurface of said beryllium substrate, said patterned masking materialfurther characterized as:(1) relatively opaque to X-ray radiation, (2)adherent to beryllium, (3) insensitive to moisture, (4) resistant tocorrosion, and (5) having a thermal coefficient of expansionsubstantially equal to said beryllium substrate and said berylliumcarrier; whereby said image mask assembly, comprising a berylliumcarrier, beryllium mask substrate, and masking material having a thermalcoefficient of expansion substantially equal to said beryllium carrierand mask substrate, will not be subject to thermal distortion due to theuse of materials having matched thermal coefficients of expansion. 10.The method of semiconductor lithography as in claim 9, wherein: saidstep of supporting said image mask orients said mask so that saidmasking material is downstream relative to said substrate.